Proton-bridged complex

Therefore, the proton is almost quantitatively transferred to the im-ine, and the heat of reaction of 140 kJ mol causes additional excitation. Thus, INC2, also called proton-bridged complex (PBC), finally decomposes to give an immonium ion, m/z 72, and propene, 42 u. 

FABMS has been shown to distinguish readily between some simple unsaturated dicarboxylic acids (96). The [M + H] ions from the Z-acids (28) are much less prone to fragment than those from the corresponding E -acids (29). The Z-acids probably form the stable proton-bridged complex (30). In contrast, protonation of the -acids on the carbon-carbon double bond becomes an energetically feasible process relative to carbonyl protonation. The ion formed can readily undergo the observed elimination of H2. 

This concerted reduction by two ferrous species eliminates H02- (or O2 ) as an intermediate and explains the weak catalysis by Cu(II) (which is strong for V([II) and V(IV) autoxidations). Weiss has suggested that the species Fe. 02.Fe may be a stable intermediate, but Wells explains the presence of two Fe(Il) species in the rate law in terms of a pre-existing dimeric form of Fe(lf) containing an H2O bridge, for which there is evidence . The reduction is completed via the Fenton reaction vide infra). The hydrogen peroxide dianion is probably never free but is protonated whilst complexed to Fe(III). 

In earlier kinetic and computer modeling [1,2, 14] the open form of CD (CDopen) was used to illustrate the adsorbed [CD - a-keto ester] complex. In this complex the quinuclidine nitrogen was involved in the interaction with the substrate directly or via a proton bridge. 

More recently, Cassidy et aid conducted additional H NMR experiments to evaluate the basicities of the dyad H57-D102 in the tetrahedral complexes of chymotrypsin with the peptidyl trifluoromethyl ketones (TFKs) N-acetyl-L-Leu-DL-Phe-CFs and A-acetyl-oL-Phe-CFs. The proton bridging His-57 and Asp-102 is part of a low-barrier hydrogen bond (LBHBs). In H NMR spectra at pH 7.0, these protons appear at 8 18.9 and 18.6 ppm. 

On protonation with HBF4 or CF3SO3H, the cobalt and rhodium complexes 120 behaved similarly to 118. However, because of the lack of a well-coordinating anion the initially formed 16-electron chalcogenolato intermediate dimerized to form the chalcogenolato-bridged complexes 121 (Scheme 29).54 170... 

The [H + ] dependence of kobs has been interpreted in terms of an acid-catalyzed and an uncatalyzed reaction path, as shown in Scheme 6. The acid-catalyzed path has been proposed to involve protonation of one hydroxo bridge to give a labile aqua-bridged intermediate. The aqua-bridged complexes have in no case been identified, but spectroscopic results indicate that such species are very strong acids with Ka3 1. The kinetic parameters calculated from the rate expression, Eq. (55), are... 

These methylene-bridged complexes III are extremely robust, air-stable substances. We have sought without success to effect insertions into the Pd-C bonds. The complexes are unreactive toward carbon monoxide (at 5 atm at 30°C) or sulfur dioxide. Reaction with methyl isocyanide or pyridine results in displacement of the terminal halide ions and produces cations that have been isolated as hexa-fluorophosphate salts [Pd2(dpm)2( -CH2)(CNCH3)2][PF6]2( (CN) = 2217 cm-1) and [Pd2(dpm)2(/Lt-CH2)(py)2][PF6]2. Treatment of III with fluoroboric or trifluoroacetic acid slowly results in the protonation of the methylene group which is converted into a terminal methyl group. The resulting brown complex, which has been isolated as its tetra-fluoroborate salt has been shown by H-l and P-31 NMR spectroscopy and X-ray crystallography to have Structure IV. 

An interesting new bridged complex, [Ru(TDBOHPP) L a L] (type H) in which the bridge L a L is 4,4 -azopyridine, has been studied in the search for molecular switches [217]. Protonation of the polymer induces partial oxidation of the Ru(II) to Ru(III) at the expense of the azo groups which are reduced to hydrazo species. Along with the formation of the Ru(II)-Ru(III) mixed valence compound a NIR intervalence band is switched on . The chemistry of these complexes is further complicated by the phenolic hydroxy groups in the porphyrin ligand which can also be deprotonated and oxidized. 

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